1. Field of the Invention
The present invention relates to a method and a unit for driving and controlling a micro machine device made up of first and second electrodes opposed to each other and a dielectric layer disposed between them, as well as a variable capacitance element and a variable capacitance switch using the method and the unit.
2. Description of the Prior Art
Recently, applications of a micro structure that is obtained by a process using a micro machine processing technique [also referred to as MEMS (Micro Electro Mechanical Systems) or MST (Micro System Technology)] to a radio communication circuit of a micro machine device have received attention (see U.S. Pat. No. 6,391,675, Japanese unexamined patent publications No. 2002-84148 and No. 2002-36197).
The micro machine device can have functions of changing capacitance or electric switch by controlling a mechanical parameter. Therefore, electric characteristics such as a loss are hardly affected by the material, so it can obtain electrically good performances more easily than a switch or a variable capacitance device using a semiconductor. Note that the micro machine device may also be called a “micro machining device”, a “micro machine element”, a “MEMS electromechanical component” or the like.
Here, a conventional structure of the variable capacitance device (variable capacitance element) realized with the micro machine device will be described with reference to FIG. 1.
In FIG. 1, the variable capacitance device is made up of a substrate 21, a lower electrode 24, a dielectric film 26 covering the lower electrode 24, a supporting film 22 and an upper electrode 25 supported by the supporting film 22, which are formed on the substrate 21. There is a space KG between the dielectric film 26 and the upper electrode 25. Capacitance CP between the lower electrode 24 and the upper electrode 25 is changed by a displacement of the upper electrode 25.
More specifically, when a voltage is applied between the lower electrode 24 and the upper electrode 25, the upper electrode 25 is attracted to the lower electrode 24 by electrostatic attraction between the electrodes so that the space KG is decreased and a value of the capacitance CP increases. The voltage that is applied between the lower electrode 24 and the upper electrode 25 is referred to as a “control voltage” or a “control voltage VC”. If the control voltage VC is sufficiently large, the upper electrode 25 is displaced until it contacts the dielectric film 26 directly so that the space KG disappears. Thus, the capacitance CP increases extremely. According to the description in U.S. Pat. No. 6,391,675, such a variable capacitance device is used as a so-called variable capacitance switch (capacitance switch).
However, the conventional variable capacitance device can be hardly controlled substantially because variation of the capacitance CP with respect to the control voltage VC exhibits hysteresis so that reproducibility is too bad to obtain desired capacitance CP precisely.
FIG. 4A shows variation of the capacitance CP when the control voltage VC is changed slowly. More specifically, the control voltage VC is increased slowly from 0 volt to 10 volts and then decreased to 0 volt, and it is further decreased to −10 volts and then increased to 0 volt. During this period hysteresis occurs, so the values of the capacitance CP at the same control voltage VC but on ascending timing and on descending timing are completely different. In addition, the capacitance CP at 0 volt of the control voltage VC is shifted to the larger side so that it has an offset value with respect to a natural value of the capacitance CP at 0 volt and cannot be reset. As a result, the capacitance CP at 0 volt of the control voltage VC is larger than the capacitance CP at 4 volts, for example.